EXCLUSIVE: Luann de Lesseps accuses Bethenny Frankel of being a mistress in explosive 'RHONY' reunion clip

Accusations will fly during Bravo's dramatic, three-part Real Housewives of New York season eight reunion! It premieres Wednesday, Aug. 31 and in ET's explosive sneak peek below, Luann de Lesseps is seen accusing Bethenny Frankel of being a mistress. "The real hypocrite in this room is Bethenny," de Lesseps says. "You're going out with a married man!"

"He's separated!" Ramona Singer yells back in Frankel's defense. The two are referring to Frankel's boyfriend, LawCash CEO Dennis Shields. Per Frankel's tweets, Shields is separated from his wife, whom the Skinnygirl mogul went to high school with. (Which is almost as strange as de Lesseps' fiancé, Tom D'Agostino, Jr., dating Singer and Sonja Morgan before her.)

But de Lesseps won't be deterred. "[Bethenny] was going out with him for a while while he was still married, living at home," she says. "Everybody knows it! ... This has been going on for a long time. So as much as you want to say that he's separated, you're separated -- that is a lie. ... That's not how it started!"

"Yes, it absolutely is," Frankel replies. De Lesseps keeps going with the allegations, telling Frankel, "I have other people that know you've slept with married men."

Frankel and de Lesseps wound up in a screaming match earlier in the season while on a weekend getaway at Dorinda Medley's Berkshires home. "You are a hypocrite," Frankel told de Lesseps during the episode. "You f**k everyone, and you pretend you don't! ... You are a slut and a liar and a hypocrite and a snake."

And things didn't end well between the two this season. In season eight's final episodes, Frankel showed de Lesseps a photo of D'Agostino, Jr. kissing another woman just days after their engagement. During the finale, de Lesseps told Frankel to stay out of her business.

The Real Housewives of New York Season 8 Reunion, Part I premieres Wednesday, Aug. 31 on Bravo.